The present embodiments relate to a local shim coil within a local coil for an imaging system.
Magnetic resonance imaging scanners (MRI scanners) for examining patients using magnetic resonance imaging are known, for example, as disclosed by DE10314215B4, U.S. Pat. No. 6,100,695, U.S. Pat. No. 6,023,167.
In MR imaging, images with a high signal-to-noise ratio (SNR) may be recorded using local coils. Local coils are antenna systems that may be provided in the immediate vicinity above (anterior) or below (posterior) a patient. During an MR measurement, excited cores induce a voltage in the individual antennae of the local coils. The voltage may be amplified by a low-noise amplifier (e.g., LNA, preamp) and forwarded in a wired manner to an electronic receiving device. High-field units (e.g., 1.5 T to 12 T and more) may be used to improve the signal-to-noise ratio even in the case of high resolution images.
The homogeneity of the B0 basic field is important in many clinical MR applications. Artifacts or distortions may occur. Some applications, such as FatSat, may no longer operate optimally in the case of differences in the homogeneity. FatSat is a method in which the frequency shift of the protons bound in the fat is used to fade out the signals of fatty tissue by a strong transmission pulse (e.g., a saturation pulse) in the case of the fat frequency. Alternatively, FatSat may use the frequency or the phase shift of the two signals to fade out the fatty tissue, in part, in image processing. Since the difference between the proton frequency in water and in fat is very slight (e.g., a few ppm of the basic field), the FatSat method is highly dependent on the spatial homogeneity of the basic field. The spatial homogeneity is currently determined by way of volumes of approximately 30×30×30 cm (+−20 cm) with up to, for example, approximately 0.5 ppm.
Distortions of the B0 basic field may occur in bodily regions due to the spatially inhomogeneous distribution of the susceptibility (mu_r or mr) of the body tissue. These distortions may be corrected by shim coils fitted in the MRI scanner system. Current shim coils are installed in the system spatially in the region of the gradient coils, relatively far away from the patient. The number of different shim coils, the arrangement and activation of which allows a certain number of degrees of freedom, may be used to compensate for B0 inhomogeneities of the usually super-conducting basic field magnets using shim currents in conventional copper coils. The number of degrees of freedom is insufficient in many systems to compensate for inhomogeneities in the region of, for example, the cervical spine (HWS). Shim coils of a higher order (e.g., a strong local field variation), which are provided in the system, may be extremely inefficient with respect to current or output versus change in B0.
Shim coils are also described in the following documents: Christoph Juchem et al., Magnetic field homogenization of the human prefrontal cortex with a set of localized electrical coils, Journal of Magnetic Resonance Imaging, MRM, 63: 171-180, 2010; GH Glover et al., Mitigation of susceptibility induced signal loss in neuroimaging using localized shim coils, MRM 2005, 243-248; R. Cusack et al., An evaluation of the use of passive shimming to improve frontal sensitivity in fMRI, Neuroimage, 2005, 24, 82-91; and J L Wilson et al., Utilization of an intra-oral diamagnetic passive shim in functional MRI scanner of the inferior frontal cortex, MRM 2003, 50, 1089-1094
According to one current solution (e.g. www.satpadinc.com/photos), gel cushions may be placed in the nape region of the neck of the patient to be examined if the shim orders are not sufficient. The residual susceptibility of such cushions, in the best case scenario, counteracts the B.sub.0 distortions such that a more homogeneous field results.